1. Field of the Invention
The present invention generally relates to a high frequency circuit, in particular, to a signal switching device that switches a transmission path to which an input signal propagates.
2. Description of the Related Art
In radio base stations, transponders, or other communication equipment used in cellular communications or satellite communications, signal switching devices are utilized for appropriately switching transmission paths of input signals. Such a signal switching device receives high frequency signals from an input circuit, selects a desired transmission path from a number of available transmission paths, and outputs the signals through the selected transmission path.
Japanese Laid Open Patent Application No. 9-275302 discloses a microwave switch, in which each of a number of micro-strip paths connected to a switching section have a part made from an oxide superconducting material, and a direct current element is provided between the switching section and the oxide superconducting part to change the oxide superconducting part from a superconducting state to a non-super conducting state (for example, a normal conducting state), or vice versa. Because of such a configuration, leakage of the microwave to the non-selected paths is reduced, improving the isolation characteristic of the microwave switch.
However, when the above technique is used to improve the isolation characteristic, degradation of signals entering the desired transmission path and loss of levels of the signals are not always reduced. In some cases, even when the leakage from the input signals to the unselected transmission paths (specifically, later stages of the paths) is zero, the signals entering the selected transmission path are strongly degraded compared to the input signals because of the length of the transmission path or other reasons. Therefore, for good quality of signal switching, not only the isolation characteristic but also the signal degradation should be considered. The related art cannot meet this requirement.
In the above signal switching device, a switching element, such as a mechanical switch or a semiconductor switch, is provided at the output of each transmission path, that is, each output of the switching device. These elements are also for preventing signals from entering the later stage circuits so as to improve the isolation characteristic. However, the reliability of a mechanical switch declines due to its switching mechanism. Although the problem related to the mechanical switch is avoidable by using a semiconductor switch, the isolation characteristic of a semiconductor switch is not as good as that of the mechanical switch. In addition, the reliability of the operation of the semiconductor switch itself has to be a concern. Further, when using the above switches, appropriate signals for controlling their switching operations have to be generated and devices capable of switching operations according to the control signals have to be configured, making a signal switching device complicated.